1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device using a photolithography technique and, more particularly, to a method of manufacturing a semiconductor device, including a step of forming wells in a semiconductor substrate using the photolithography technique.
2. Description of the Background Art
A so-called lithography technique of forming or mass producing patterns of miniaturized devices or circuits by making use of materials sensitive to light or radiation, has been used in a process of manufacturing semiconductor devices, typically semiconductor integrated circuits.
A process of forming a triple well structure as one example of the process of manufacturing a semiconductor devices using the photolithography technique will be described below. The triple well structure is characterized in that an N well and a P well are provided in a surface layer of a semiconductor substrate in such a manner as to be adjacent to each other, and a bottom N well (hereinafter, referred to as xe2x80x9cBN wellxe2x80x9d) is provided on the underside of the P well. The BN well is provided for preventing latch up caused by a pnpn thyristor circuit formed when a transistor is formed on the wells provided in the surface layer of the semiconductor substrate. In addition, the BN well may be formed not on the underside of the P well but on the underside of the N well.
FIGS. 20 to 23 are partial sectional views illustrating steps of forming the above-described triple well structure. First, as shown in FIG. 20, an underlying film 102, typically a silicon oxide film is formed on a semiconductor substrate 101 and an N well 103 is formed in a surface layer of the semiconductor substrate 101 using the conventional lithography technique. Then, an acid generating chemical amplification resist film 106 of positive type is formed on the semiconductor substrate 101.
The acid generating chemical amplification resist film 106 is made from a resist containing an acid generating agent and an alkali-soluble resin to which a solution suppressing base is introduced. In a case where the positive type acid generating chemical amplification resist film is used, when the resist film is exposed to exposure rays, acid is generated in the resist film and the solution suppressing base is decomposed by the acid functioning as a catalyst, with a result that the resist film is made alkali-soluble; while, when the resist film is not exposed to exposure rays, since acid is not generated in the resist film, the resist film is left as alkali-insoluble.
Next, as shown in FIG. 21, using a mask having a non-mask region and a mask region, a portion of the resist film 106 positioned under the non-mask region of the mask is irradiated with monochromatic rays, to be made alkali-soluble. Then, the alkali-soluble portion of the resist film 106 is removed by subjecting to baking treatment and development treatment, thereby forming a resist mask 106.
At this time, there remains the resist film 106 under the mask region of the mask. Because of the effects of standing waves formed by monochromatic rays used as exposure rays as well as of such an uneven intensity distribution of the exposure rays that the intensity of the incident rays at the lower portion of the resist film is smaller than that at the upper portion of the resist film, as shown in FIG. 21, the side wall of the remaining portion of the resist film 106 is formed with a taper portion 106a, that is, a portion horizontally broadened toward the bottom. In this specification, the horizontal distance between the upper edge of the side wall and the bottom edge of the portion horizontally broadened from the side wall is hereinafter called the width of the taper. That is to say, the distance W shown in FIG. 21 is called the width of the taper.
After formation of such a resist mask 106, as shown in FIG. 22, ions of a P-type impurity such as boron are implanted in the surface layer of the substrate 101 via the resist mask 106, to form a P well 104, and then ions of an N-type impurity such as phosphorous are implanted in the surface layer of the substrate 101 at a higher energy using the resist mask 106, to form a BN well 105 on the underside of the P well 104. The resist mask 106 is then removed, to form a triple well structure shown in FIG. 23. Here, with respect to the BN well 105 thus formed, since the taper portion 106a is formed on the side wall of the resist mask 106 as described above, a portion curved upwardly (called a BN extension portion 105 is formed at the end portion, positioned under the taper 106a, of the BN well 105 as shown in FIG. 22.
At the above-described lithography step, as shown in FIG. 24, at a portion of the non-mask region where is separated a specific distance from the edge of the mask region of a mask 107, the exposure rays are allowed to pass therethrough in all directions without any cutoff by the mask 107. Accordingly, incident rays (for example, incident rays 108a and 108b) reach a bottom region 106b of the resist film 106 positioned under the above portion of the non-mask region.
On the other hand, at a portion of the non-mask region, near the edge of the mask region of the mask 107, the exposure rays traveling from the mask region side are not allowed to pass therethrough because of cutoff by the mask 107. Accordingly, the exposure rays traveling from the mask region side never reach a bottom region 106c of the resist film 106 positioned under the above portion of the non-mask region.
Further, since each incident ray radiated on the resist film 106 is absorbed in the resist film 106, the intensity of the incident ray becomes decayed along with the advance of the incident ray in the resist film 106. After all, the intensity of the incident ray, when it reaches the bottom of the resist film 106, becomes quite weak. While part of incident rays reaching the bottom of the resist film 106 is reflected thereby, the intensities of the reflected rays are very weak because the incident rays are decayed at the bottom of the resist film 106.
As is apparent from the above description, the bottom region 106b positioned under the portion of the non-mask region separated the specific distance from the edge of the mask region of the mask 107 receives the exposure rays in all directions although the intensities thereof are weaker than those at the upper portion of the resist film 106, and consequently the bottom region 106b of the resist film 106c an obtain the intensities of the exposure rays which are large enough to make the resist film 106 alkali-soluble.
On the contrary, the bottom region 106c positioned under the portion of the non-mask region near the edge of the mask region of the mask 107 receives only the exposure rays traveling in the limited directions, the intensities of the exposure rays being, as described above, weaker than those at the upper portion of the resist film 106, and consequently the bottom region 106c of the resist film 106 cannot obtain the intensities of the exposure rays which are enough to make the resist film 106 alkali-soluble.
As a result, the bottom region 106c of the resist film 106 positioned under the portion of the non-mask region near the edge of the mask region of the mask 107 is left as alkali-insoluble, and is not removed after development treatment. In this way, as shown in FIG. 21, the side wall of the resist film 106 is formed with the portion horizontally broadened toward the bottom, that is, the taper portion 106a. 
When the resist film is irradiated with monochromatic rays, standing waves occur in the resist film by interference between the incident rays and the reflected rays from the surface of the semiconductor substrate. Since portions of the resist film corresponding to the middle of the nodes (called antinodes hereunder) of the standing waves are strongly sensitized and portions of the resist film corresponding to the nodes of the standing waves are weakly sensitized, the light exposure is repeatedly changed depending on the standing waves. As a result, irregularities are formed on the side wall of the resist mask depending on the unevenness of the light exposure.
A method of reducing such a width of the taper has been disclosed in Japanese Patent Laid-open No. Hei 4-239116, in which an underlying film having a thickness equivalent of xcex/4 (xcex: wavelength of the exposure rays) is formed on a semiconductor substrate. The underlying film sets the positions of an antinode of standing waves at the bottom of the resist film when incident rays are radiated at the right angle on the semiconductor substrate, thereby reducing the width of the taper formed on the side wall of the resist mask. Such a method, however, fails to obtain a sufficient effect because it directs attention only to the incident rays radiated at right angles on the substrate and it does not take the incident rays made incident obliquely on the substrate into account.
Further, it may be considered to set the best focus at the bottom of a resist film for reducing the width of the taper. According to this method, the intensities of exposure rays at the bottom of the resist film can be surely increased but the intensities of the exposure rays at the surface of the resist film are decreased because the surface of the resist film becomes defocused. In the case of a resist film having a large thickness, this method causes a problem that a portion to be exposed, near the surface of the resist film, cannot be sufficiently exposed to exposure rays, thus being left as alkali-insoluble. Accordingly, to develop the entire resist film so as to correspond to the mask, the best focus is generally set at a portion near the center of the thickness of a resist film. As a result, a taper portion may be formed on the side wall of a resist mask obtained from the resist film for the above reason.
If a taper portion is formed on the side wall of a resist mask, the shape of the actually formed resist mask is different from that of the designed resist mask to be produced. Namely, there is caused a problem such as formation of the above-described BN extension portion, resulting in that an impurity region cannot be formed as designed.
Another problem of the above-described BN extension portion is that a between-well breakdown voltage in the triple well structure differs depending on the presence or absence of the BN extension.
FIG. 25 is a view showing a triple well structure in which a BN well 105 is formed in a semiconductor substrate and a P+ contact portion 109 is provided in an N well 103. FIG. 26 is a graph showing a relationship between the between-well breakdown voltage and the presence or absence of a BN extension portion on the BN well 105. In FIG. 26, the ordinate designates the breakdown voltage; the abscissa designates a distance (xcexcm) between the contact portion and an adjacent portion of the wells 103 and 104; a solid line shows the breakdown voltage of the triple well structure in which the BN extension portion is formed; and a broken line shows the breakdown voltage of the triple well structure in which the BN extension portion is not formed.
As is apparent from FIG. 26, the between-well breakdown voltage differs depending on the presence or absence of the BN extension portion. Specifically, the minimum distance between the contact portion and the adjacent portion of the wells for assuring the maximum breakdown voltage becomes al (about 1.4 xcexcm) for the triple well structure in which the BN extension portion is formed, and becomes a2 (about 0.4 xcexcm) for the triple well structure in which the BN extension portion is not formed.
As described above, to assure the maximum breakdown voltage, the above minimum distance for the triple well structure in which the BN extension portion is formed must be made larger than that for the triple well structure in which the BN extension portion is not formed by a value of a1xe2x88x92a2 (about 1.4 xcexcmxe2x88x92about 0.4 xcexcm=1.0 xcexcm in the above example). Accordingly, in the case of manufacturing a semiconductor device having a triple well structure required to assure the maximum breakdown voltage, if the BN extension portion is formed in the structure, the distance from the contact portion must be made larger than that for the case in which the BN extension portion is not formed, so that the chip area must be made correspondingly larger. This causes reduction in the theoretical number of chips to be manufactured per wafer, thereby increasing the manufacturing cost.
FIG. 27 shows a triple well structure in which a P well 104 is surrounded by N wells 103 as well as BN wells 105 are formed under the N wells 103, wherein BN extension portions 105a are formed on side walls of the BN wells 105, that is, on both sides of the P well 104. Even in this structure, like the above-described triple well structure, the between-well breakdown voltage for the structure in which the BN extension portions are formed is smaller than that for the structure in which they are not formed, and consequently, to assure the maximum between-well breakdown voltage, a distance A across the P well 104 for the structure in which the BN extension portions are formed must be made larger than that for the structure in which they are not formed.
FIG. 28 is a graph showing a relationship between the between-well breakdown voltage and the presence or absence of the BN extension portions in the structure shown in FIG. 27. In FIG. 28, the ordinate designates the breakdown voltage; the abscissa designates a distance (xcexcm) between the contact portion and the adjacent portion of the wells; and the negative region of the distance A indicates a length of an area in which both the N wells 103 are overlapped to each other. In this negative region of the distance A, therefore, the entire region of the P well 104 is covered with both the N wells 103. In this graph, a solid line shows the breakdown voltage of the structure in which the BN extension portions are formed and a broken line shows the breakdown voltage of the structure in which the BN extension portions are not formed.
As is apparent from FIG. 28, the minimum distance between the contact portion and the adjacent portion of the wells for assuring the maximum breakdown voltage becomes al (about 2.4 xcexcm) for the structure in which the BN extension portions are formed, and becomes a2 (about 1.4 xcexcm) for the structure in which the BN extension portions are not formed.
As described above, to assure the maximum breakdown voltage, the above minimum distance for the structure in which the BN extension portions are formed must be made larger than that for the structure in which the BN extension portions are not formed by a value of a1xe2x88x92a2 (about 2.4 xcexcmxe2x88x92about 1.4 xcexcm=1.0 xcexcm in the above example).
An object of the present invention is to provide a method of manufacturing a semiconductor device, which is intended to reduce the width of a taper formed on a side wall of a resist mask by utilizing a resist film having a property that a portion reacting with acid is made water-insoluble.
Another object of the present invention is to provide a method of manufacturing a semiconductor device, which enables accurate lithography by suppressing the width of a taper formed on a side wall of a resist film.
A further object of the present invention is to provide a method of manufacturing a semiconductor device, which is capable of reducing a between-well distance by preventing an extension portion from being formed on a side wall of a BN well.
The above objects of the present invention are achieved by a method of manufacturing a semiconductor device, including the steps of: forming a first resist mask having an opening portion of a specific pattern on a semiconductor substrate, the opening portion having a side wall formed with a taper portion; forming a water-soluble resist film on the first resist mask in such a manner as to cover at least the taper portion, the water-soluble resist film being made water-insoluble when it reacts with acid; allowing the water-soluble resist film to react with acid, to form a water-insoluble portion on the taper portion; removing the water-soluble resist film while leaving the water-insoluble portion, to form a second resist mask composed of the water-insoluble portion formed on the taper portion and the first resist mask; and implanting an impurity in the semiconductor substrate via the second resist mask, to form an impurity region in the semiconductor substrate.
Since the thickness of the remaining water-insoluble portion of the resist film is dependent on the shape of the under layer, i.e., the first resist mask, the thickness of the remaining portion of the resist film is larger at a location on the taper than at a location on the flat surface. Thus, According to the present invention, it is possible to make small the width of a taper formed on a side wall of the remaining portion of the resist film, i.e., on a side wall of the second resist.
The above objects of the present invention are also achieved by a method of manufacturing a semiconductor device, including the steps of: forming an underlying film on a semiconductor substrate; forming a resist film on the underlying film; exposing the resist film to exposure rays having a specific wavelength, to form a mask having a specific pattern; and implanting an impurity in the semiconductor substrate via the mask, to form an impurity region in the semiconductor substrate; wherein the thickness of the underlying layer is set such that upon exposure, a phase of an exposure ray reflected from the front surface of the underlying film in a specific direction is matched, at the front surface of the underlying film, with a phase of an exposure ray having been reflected from the bottom surface of the underlying film and passing through the front surface of the underlying film in the specific direction.
According to the present invention, it is possible to enhance the intensity of the exposure rays on the front surface of the underlying film, whereby the width of a taper formed on the side wall of the resist mask is sufficiently suppressed.
The above objects of the present invention are further achieved by a method of manufacturing a semiconductor device, including the steps of: forming an underlying film on a semiconductor substrate; forming a resist film on the underlying film; exposing the resist film to exposure rays, to form a mask having a specific pattern; and implanting an impurity in the semiconductor substrate via the mask, to form an impurity region in the semiconductor substrate; wherein the reflectance of the underlying film is set such that the bottom portion of the resist film is exposed to the exposure rays reflected from the underlying film upon exposure.
According to the present invention, it is possible to enhance the intensity of the exposure rays on the bottom portion of the resist film, and hence to suppress the width of a taper formed on a side wall of the resist mask.
The present invention is obtained as a result of experiments which are performed directing an attention to the property of the above described resist film that water-insoluble crosslinking portion is formed when crosslinking
Other objects and further features of the present invention will be apparent from the following detailed description when read in conjunction with the accompanying drawings.